Hydrogel polymers have been used for many years for a wide variety of medical devices. Two types of medical devices where hydrogel polymers are particularly well suited and well established are soft contact lenses and intraocular lenses.
As water content of a hydrogel polymer increases, its properties change dramatically. The desired water content is driven by the polymers end use. Soft contact lens polymers have advantageous properties as the water content increases.
Comfort and oxygen permeability increase as water content of a contact hydrogel increases but almost all hydrogel polymers lose water when on the eye at a rate that increases with increasing water content. A typical soft contact lens polymer with a saturated water content of 60% will have only 54-55% water content after a few hours on the eye. This loss of water content creates instability whereas the fit becomes much tighter and lens comfortable to the user. Furthermore, the loss of water causes the refractive index to increase, thereby increasing the lens power. These are typical properties of almost all soft contact lenses and cause the lens to be less comfortable towards the end of the day or wearing cycle. One of the only commercially-available non-ionic soft contact lens polymers that do not lose water on the eye is hioxifilcon, a copolymer of 2-Hydroxyethylmethacrylate and Glycerol Methacrylate. See, e.g., U.S. Pat. No. 5,532,289. This copolymer family can be effectively formulated for soft contact lenses with water content ranging from 49% to 66%. Glycerol Methacrylate homopolymer can reach 74-75% water content but generally has mechanical disadvantages when used in devices such as contact lenses. There is a need, however, for very high water content hydrogels, 70-85%, that exhibit high stability to water loss on the eye. There is also a need for very high water content polymers that have increased hydrolytic stability to withstand high temperatures such as autoclave temperatures, 123° C. or even higher temperatures while in an aqueous environment.